72 THE SOIL. 



treatment of the soil and the influence of the weather 

 serve to strengthen the amses which bring about the 

 disintegration and decomposition of the mmerals, and the 

 uniform distribution of the elements of food contained 

 in them and rendered soluble. The elements chemically 

 combined in the minerals are released from that com- 

 bination, and in the arable soil gradually resulting from 

 this decomposition acquire the form in which they are 

 available as food for plants. It is evident that only by 

 degrees the rough ground can attain the properties of arable 

 soil, and that the time required for this change depends 

 upon the quantity of nutritive substances present, and 

 upon the obstacles which oppose their distribution, or 

 their disintegration and decomposition. The perennial 

 plants, and particularly the so-called weeds, consuming in 

 proportion to the time less food, and absorbing longer, 

 -will always thrive on a soil of this description long before 

 annual or summer plants, which in their shorter period 

 of vegetation require a far larger amount of nutritive 

 substances for their full developement. 



The longer a soil is under cultivation, tlie more it 

 becomes suited for the growth of summer plants, from 

 the recurrence and operation of the causes by which the 

 nutritive substances are converted from a state of chemical 

 into one of physical combination. To be productive, in 

 the fullest sense of the term, a soil must be able to afford 

 food at all points in contact with the roots of the plants ; 

 and, however small the quantity of this food may be, it 

 must necessarily be distributed through every part of the 

 soil. 



The power of the soil to nourish cultivated plants is 

 therefore in exact proportion to the quantity of nutritive 

 substances which it contains in a state of physical satura- 

 tion.. The quantity of the other elements in a state of 



